Lumps, Bumps and Growths - Oh, my!

Skin Cancer in Horses

            "What do you think it is, doc?" This timeless question is one of my favorites. It's almost inevitable in most situations, whether it be regarding a vomiting dog or a lame horse. Sometimes, I might actually have an answer for you (ie your dog is vomiting because it ate three pairs of underwear and half the sofa). But when it comes to many lumps or bumps on the skin, I really want to yell out, "Damnit, Jim, I'm a doctor, not a pathologist!" a la Star Trek's Dr. McCoy, but that probably wouldn't go over so well with most clients. You see, there is no one single presentation or appearance for many skin tumors, so there is no way to tell one lump from another just by looking at it. Yes, we might have suspicions based on appearance, location or age of the animal, but you can never be 100% sure until you do some testing. And testing often requires sending the pathologist a few samples of the lump, either slides from a fine needle aspirate or the entire growth after surgical removal. Show me a lump on your dog or horse and I will most likely respond: "It can be difficult to say based on appearance. I think it might by XYZ and it could be benign/malignant. Let's consider aspirating/removing/monitoring it." Testing is a very important part of diagnosis. Pathologists receive extensive training to identify cell types, much more training than us general practitioners. We can make some good educated guesses however, especially in the three most common types of equine skin tumors covered in this article - melanomas, sarcoids and squamous cell carcinomas.

      Cutaneous melanomas are very common skin tumors we see in horses. Primarily, they are seen in grey or white horses and in fact, more than 80% of grey horses older than 15 years will develop melanoma. They commonly occur under the tail, on lips, around the eyes or external genitalia. Melanoma presents in several disease subtypes, ranging from slow growing benign lesions to highly malignant invasive lesions. Thankfully, approximately 50% of all melanomas are considered benign. A presumptive diagnosis can be made based on appearance - firm, flat, solitary or multiple hyperpigmented cutaneous masses giving skin a verrucous (or cobblestone) appearance. Histopathology is useful in determining clinical behavior and risk of metastasis, so surgical removal or fine needle aspirates may be recommended depending on appearance and location. And even benign lesions can cause problems depending on location. I have seen several horses with so many melanomas under their tail that they actually had trouble defecating! Obviously, that is not a good situation for any horse, and we usually try to recommend treatment before melanomas progress that far.

      There is no uniform standard therapy for melanoma and the goal of treatment is often to simply manage the disease. Surgery is an option for some melanomas if they are small and discreet, but it is often contraindicated around the anus or tail due to potential for aggressive regrowth. Chemotherapy can be successful for small lesions and is more successful following surgical debulking of large lesions. Various immunotherapies may also be viable treatment options, including oral cimetidine or whole-cell melanoma vaccine. Prognosis depends on size, location and type.

      While melanoma is very common in grey horses, sarcoids are actually the most common skin cancer seen in horses overall. There is no association with skin color. They are relatively more common in Quarter Horses, Appaloosas and Arabians, but any breed is susceptible. Appearances vary as there are six subtypes - occult, verrucous (warty), nodular, fibroblastic, mixed and malevolent. While typically benign, any subtype is capable of transforming into a more malignant form. Lesions can occur anywhere on the body, but are more commonly found on the face (ears, muzzle and around the eyes), distal limbs, neck, ventral abdomen and any area of previous injury or scarring. So, can sarcoids can look like anything and pop up anywhere? Yep.

      How do you figure out if a lump is a sarcoid versus some other lesion? Biopsy can give you a definitive diagnosis, but because of the variable appearance, pathologic identification can be difficult. Most sarcoids also carry a bovine papilloma virus (BPV), so it is possible to test lesions for BPV DNA. It is unclear what role BPV plays in equine sarcoids at this time. Often, a veterinarian will simply suspect sarcoids by location and appearance, but this is not an exact method of diagnosis.

      Treatment is equally inexact. There are a large number of treatment options, including surgical removal, cryotherapy, immunotherapy, chemotherapy or bloodroot extract (ie Xterra product).  Treatment often depends on the type of lesion, location, size, or availability of treatment modality. If your horse does have sarcoids, it is best to speak with your veterinarian to discuss what treatment, if any, should be pursued. No treatment is 100% effective and re-treatment is often necessary. Recurrance is roughly 20-30%, regardless of therapy. Obviously, this can make sarcoids very frustrating to treat! While sarcoids are rarely life-threatening, they may pose significant aesthetic and performance-limiting problems.

      Squamous cell carcinomas (SCC), on the other hand, pose a much greater risk to a horse as all SCC are malignant. This is the second most common type of tumor in horses overall and is more commonly seen in Draft horses, Appaloosas and Paints. Horses living at higher elevations with increased sunlight exposure do have a higher incidence of SCC. This is thought to occur due to increased UV light radiation promoting the development of this type of skin cancer. SCC has also been reported to develop at chronic wounds, burn sites or other sites of scarring.

      Again, location of skin tumors can be a big clue in diagnosing a SCC. It most commonly affects the skin, external genitalia (e.g. penis), nasal and paranasal sinus, eye and periocular (around the eye) areas. Signs relate to location. For example, purulent nasal discharge is often seen in nasal sinus SCC or difficulty urinating can be seen with SCC of the penis. Any erosive, erythematous (reddened), or raised mass is a SCC suspect. Ocular SCC behaves differently than other types and prognosis is generally worse. Ocular discharge, squinting and a tan to reddish raised nodular mass or plaque indicate a possible ocular SCC. The most common type of SCC that we see in our practice is on the prepuce or penis of older geldings. A biopsy is always recommended, not only to diagnose SCC but to determine proper surgical planning (wider excision is needed for malignant tumors versus benign lesions).

      The treatment of choice is usually surgery, although some locations are not amenable to surgical removal (ie. periocular lesions). Believe it or not - penile amputation is surprisingly well-tolerated with few long-term complications and low recurrence rates. You fellows reading this are probably cringing at that thought! Adjunctive therapy is used in cases where surgical excision is not practical or it is incomplete. Radiation or chemotherapy is common, either with or without surgical excision first. 5-fluorouracil (5-FU) is a very common chemotherapy agent and often used both topically or injected directly into a lesion. Other therapies include immunotherapy, cryotherapy, or intralesional injection of several agents. When therapy can adequately control the SCC, long term prognosis can be good (2+ years disease-free) in 60-80% of affected horses. Prognosis is obviously more guarded in cases where the lesions cannot be surgically removed. Smaller lesions are treated more successfully and with lower recurrence rates.

      So what is the morale of the story, kids? Don't ignore those lumps and bumps on your horses! It can be difficult to tell if a lesion is benign or malignant just by looking at it. All of the above skin cancers have a wide variety of appearances. A seemingly innocuous bump can transform into a malignant cancer. Veterinarians are not pathologists! We can make educated guesses based on appearance, location and other factors, but testing is a very important part of identifying if a lesion is concerning or not.